1. Field of the Invention
The present invention relates to rear-focus zoom lenses (lens devices), and more specifically to a zoom lens used in a shooting apparatus, such as a television camera and a video camera.
2. Description of the Related Art
Shooting apparatuses, such as television cameras and video cameras, require large-aperture zoom lenses which provide high variable power ratios and high optical performance. In particular, color television cameras for broadcasting are preferably structured such that they can be easily operated and maneuvered, and accordingly, small solid-state imaging devices having a size of ⅔ or ½ inches are generally used. Imaging devices have a substantially constant resolution over the entire imaging area thereof, and it is therefore necessary that zoom lenses also have a substantially constant resolution from the center to the periphery of a view area.
Accordingly, in zoom lenses, it is important to accurately correct not only aberrations like spherical aberration, coma aberration, etc., but also aberrations asymmetric with respect to an optical axis, such as eccentric coma aberration which is generated due to manufacturing errors between components, so that high optical performance is obtained over the entire view area.
On the other hand, so-called four-unit zoom lenses including a first lens unit which is a positive lens unit and which is fixed during variation of magnification, a second lens unit (variator lens unit) which is a negative lens unit and which moves during the variation of magnification, a third lens unit (compensator lens unit) which is a negative lens unit and which performs image-plane correction during the variation of magnification, and a fourth lens unit (focus lens unit) which is a positive lens unit having an image-forming function, which is used for focusing, and which is fixed during the variation of magnification, in that order from an object side, are known in the art. The four-unit zoom lenses are small, and magnifications and performance thereof can be easily improved. Accordingly, the four-unit zoom lenses are often used as zoom lenses for broadcasting and professional purposes.
Examples of four-unit zoom lenses are disclosed in Japanese Patent Laid-Open No. 1-120522 (see line 16 in the lower right column of page 2 to line 2 in the upper left column of page 3 and FIG. 1), Japanese Patent Laid-Open No. 1-284818 (see lines 12 to 18 in the lower left column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 2-100011 (see lines 11 to 19 in the lower right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 2-118510 (see lines 1 to 10 in the upper right column of page 3 and FIGS. 1(A) and 1(B)), Japanese Patent Laid-Open No. 2-208618 (see the third line from the bottom in the upper right column to line 7 in the lower left column of page 3 and FIG. 1), Japanese Patent Laid-Open No. 2-208619 (see lines 11 to 19 in the lower right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 2-208620 (see lines 11 to 19 in the lower right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 3-123310 (see lines 9 to 17 in the lower right column of page 2 and FIG. 1), Japanese Patent Laid-Open No. 3-145615 (see lines 3 to 11 in the lower right column of page 2 and FIGS. 1 to 8), and Japanese Patent Laid-Open No. 4-138407 (see lines 2 to 10 in the lower right column of page 2 and FIG. 1).
In addition, three-unit zoom lenses including a first lens unit which is a positive lens unit and which is fixed during the variation of magnification, a second lens unit (variator lens unit) which is a negative power and which moves during the variation of magnification, and a third lens unit which is a positive lens unit having an image-forming function and which is fixed during the variation of magnification, in that order from an object side, are also known in the art. The third lens unit includes a lens sub-unit (focus lens sub-unit) having a function of both focusing and performing image-plane correction during the variation of magnification. The three-unit zoom lenses have a relatively simple structure, and are therefore often used in commercial video cameras and the like.
Examples of three-unit zoom lenses having a structure in which a fixed lens sub-unit is provided at a position nearer to an image plane than the focus lens sub-unit to reduce the size and weight and to improve the performance are disclosed in, for example, Japanese Patent Laid-Open No. 8-5913 (see paragraphs 0013 to 0014 and FIGS. 2 to 14), Japanese Patent Laid-Open No. 9-159917 (see paragraphs 0012 to 0013 and FIG. 1), Japanese Patent No. 3097399 (see paragraphs 0014 to 0019 and FIGS. 1 to 4), and Japanese Patent Laid-Open No. 2000-284173 (see paragraphs 0032 to 0035 and FIG. 1).
In optical systems including a plurality of lens units as described above, the optical performance may be largely influenced by inclination, parallel eccentricity, etc., of each lens unit with respect to an optical axis caused by differences in precision between lens pieces and components in a lens barrel. In order to adjust the inclination, parallel eccentricity, etc., some zoom lenses include a mechanism for making one of the lens units eccentric in parallel or inclined with respect to the optical axis. The adjusting lens unit has a sufficient sensitivity to eccentric coma aberration, etc.
On the other hand, rear-focus zoom lenses in which a focus lens unit is disposed at a position nearer to an image plane than a variator lens unit are often used as auto-focus zoom lenses since the size and weight of the focus lens unit can be reduced.
In the four-unit zoom lenses disclosed in the above-mentioned publications, four lens units consisting of positive, negative, negative, and positive lens units are arranged in that order from the object side. However, since a converted inclination angle of incidence of an on-axis light ray on a side of the fourth lens unit which faces the image plane is large, when focusing is performed at the side facing the image plane, an incidence height of the on-axis light ray largely varies and variations in on-axis aberrations, such as spherical aberration and on-axis chromatic aberration, are increased. In addition, since the refractive power at a side of the fourth lens unit which faces the object is reduced, an additional positive lens must be disposed at a position nearer to the object than a diaphragm for causing a divergent light beam from the third lens unit to converge. In addition, the refractive power of the third lens unit must be reduced to reduce the divergence from the third lens unit. As a result, the amount of movement of the third lens unit is increased and the overall length of the zoom lens is also increased.
In addition, in the three-unit zoom lenses disclosed in the above-mentioned publications, three lens units consisting of positive, negative, and positive lens units are arranged in that order from the object side. However, since a converted inclination angle of incidence of an on-axis light ray on the focus lens sub-unit included in the third lens unit is large, when focusing is performed at the focus lens sub-unit, an incidence height of the on-axis light ray largely varies and variations in the on-axis aberrations, such as spherical aberration and on-axis chromatic aberration, are increased. In addition, according to Japanese Patent Laid-Open No. 2000-284173, a fixed lens sub-unit having a negative refractive power is disposed at a position nearer to the image plane than the focus lens sub-unit included in the third lens unit. Accordingly, the refractive power of the focus lens sub-unit is increased and the absolute values and variations in the on-axis aberrations, such as spherical aberration and on-axis chromatic aberration, are increased.
In addition, in rear-focus zoom lenses, when performance degradation at a wide-angle end due to manufacturing errors is to be corrected, rear-focus lens units are generally not suitable for use as adjustment lens units because they move.
In the three-unit zoom lenses commonly used in commercial video cameras and the like, eccentric aberration is often corrected by adjusting a fixed lens sub-unit included in the third lens unit. Alternatively, adjustment lens units for correcting the eccentric aberration may not be provided in view of the optical performance required by users and the product cost.
In comparison, in the four-unit zoom lenses commonly used for broadcasting and professional purposes, users require high optical performance, and therefore it is necessary to correct the eccentric aberration. In addition, an adjustment lens unit for correcting the eccentric aberration is preferably provided in the fourth lens unit as a fixed lens sub-unit. However, a diaphragm unit and a drive unit for electrically controlling the variation of magnification are disposed near the side of the fourth lens unit which faces the object, and it is not preferable to dispose the adjustment lens sub-unit at this position since the structure becomes complex.